DIAGNOSIS The Dialogue Mode is likely already active. The button toggles the mode on or off; there's no distinct indicator light for it on the soundbar itself.
FIX First, identify if it's on. Play content with spoken dialogue. Press the 'Dialogue' button on the remote once. Listen for a clear change in vocal clarity and brightness. If you hear a change, it was off and is now on. If you hear no change, press it again—it may have been on and you just turned it off.
IF THAT DIDN'T WORK The core issue may be your source or placement. Ensure your TV is outputting a standard stereo or Dolby Digital signal, not a complex surround format the bar must downmix. Also, center the soundbar directly under your TV for the best vocal projection.
HEADS UP Dialogue Mode primarily boosts the mid-range frequencies where speech lives. It won't fix badly mixed audio from the source. Try different shows or inputs to test.